JPH0717768A - Silicon nitride ceramic substrate and use thereof - Google Patents

Abstract

PURPOSE:To provide a silicon nitride ceramic substrate having low thermal resistance and suitable for use as a substrate for a thyristor and a ceramic substrate with a copper circuit remarkably excellent in durability to thermal shocks and heat history. CONSTITUTION:The objective silicon nitride ceramic substrate has <=0.20 deg.C/W thermal resistance. A copper circuit is, formed on one side of this silicon nitride ceramic substrate and a copper sheet having an area nearly equal to that of the substrate is joined to the other side to obtain the objective ceramic substrate with the copper circuit.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention uses a silicon nitride ceramic substrate suitable for use as a substrate for thyristors, which is a core of power devices corresponding to the power control section of electronic parts, and the silicon nitride ceramic substrate. The present invention relates to a ceramic substrate having a copper circuit formed of

[0002]

2. Description of the Related Art In recent years, as industrial equipment such as robots and motors have become higher in performance, thyristors have been developed into
There are various types such as gate turn-off (GTO) type, and due to the recent progress in design technology and manufacturing process technology of the device structure, high breakdown voltage and large current have been developed, and heat generated from semiconductor devices has been increasing. ing. In order to efficiently dissipate this heat, various methods have been conventionally used for the thyristor substrate. Especially recently, since good heat conductive ceramics substrates have become available,
A structure is also being adopted in which a metal plate such as a copper plate is bonded onto a substrate, and after forming a circuit, a semiconductor element is mounted as it is or after a treatment such as plating is performed.

It is expected that the use of the element using such a thyristor substrate will be expanded to various home electric appliances such as various machine tools, washing machines, microwave ovens, from robots and motors.

As a substrate for thyristors, an aluminum nitride substrate has been receiving attention instead of a conventional alumina substrate. As a method for joining the aluminum nitride substrate and the copper plate, an active metal brazing method in which a brazing material containing an active metal is interposed between the copper plate and the aluminum nitride substrate and heat treatment is performed (for example, JP-A-60-177634). Alternatively, there is a DBC method (for example, Japanese Patent Laid-Open No. 56-163093) in which an aluminum nitride substrate whose surface is oxidized and a copper plate are heated and bonded at a melting point of copper or lower and a eutectic temperature of Cu-O or higher.

[0005]

The characteristics that the thyristor substrate must have are heat dissipation, mechanical strength, reliability of bonding, etc. Of these, the heat dissipation is largely dependent on the characteristics of the substrate material. Is. Heretofore, as a substrate of this type, ceramic substrates such as beryllium oxide, aluminum nitride, and silicon carbide have been studied from the viewpoint of high thermal conductivity, but these have advantages and disadvantages. For example, beryllium oxide has excellent thermal conductivity but is toxic, and aluminum nitride is technically difficult to bring the thermal conductivity close to the theoretical value, resulting in high cost. More importantly, since many thyristor substrates are large in size, mechanical strength of these ceramic substrates is insufficient and reliability is questioned, and the emergence of other materials is awaited.

On the other hand, in recent years, since the electric power loaded on the thyristor has advanced to a region exceeding several kW, the thermal resistance value at the second or third digit after the decimal point, which has not been so important so far, is a problem. Came to be said. In other words, even if a ceramics substrate is sufficiently filled with properties such as strength and thermal conductivity, when it is modularized, the heat radiation from the element cannot be radiated due to defects such as metallization and voids generated at the bonding interface with the copper plate. There is a problem that it is sufficient, that is, the thermal resistance does not decrease as calculated. Therefore, it has been required to manage the thermal resistance so as not to make a difference from the case of actual use as much as possible.

From the above, the requirements for a thyristor substrate today are high strength, excellent thermal conductivity or low thermal resistance, and good bondability with a copper plate.

As a result of various investigations in order to meet the above demands, the present inventors have focused their attention on silicon nitride as a material for a ceramic substrate and made it thinner to reduce the thermal resistance from the structural viewpoint of the substrate. In this case, there is no problem with the expected reliability of the substrate, in other words, a ceramic substrate with low thermal resistance and sufficient durability against thermal shock and thermal history. The present invention has been completed and the present invention has been completed.

[0009]

That is, the present invention provides a silicon nitride ceramic substrate having a thermal resistance of 0.20 ° C./W or less, and a copper nitride on one surface of the silicon nitride ceramic substrate. A ceramic substrate having a copper circuit, wherein a circuit is formed and a copper plate having an area substantially equal to that of the silicon nitride ceramic substrate is bonded to the opposite surface.

The present invention will be described in more detail below. Generally, the relationship between the thermal conductivity and the thermal resistance of a ceramic substrate can be expressed by the equation (1). R _P = k · (1 / λ) · (t / S) (1) R _P : Thermal resistance of ceramic substrate (° C./W) k: Coefficient λ: Thermal conductivity (W / mK) t: Thickness of ceramics substrate (m) S: Area of ceramics substrate (m ² )

In order to reduce the thermal resistance of the ceramic substrate from the equation (1), three methods can be considered: increasing the thermal conductivity, reducing the thickness, and increasing the area. Of these, the thermal conductivity is determined by the material selected, and it is technically difficult and limited to improve the thermal conductivity with the same material. Further, the area of the ceramic substrate can be easily changed, but cannot be changed so freely because it is limited by other technical aspects such as the size of the module. Therefore, the present inventors have paid attention to reducing the thickness of the silicon nitride ceramic substrate.

Usually, the ceramic substrate having a copper circuit used in this field has a thickness of 0.635 to 1 mm when the substrate is aluminum nitride, for example. Here, for example, if the thickness is set to 60%, the thermal resistance is also 60%. That is, there is an effect equivalent to that in which the thermal conductivity of the aluminum nitride substrate is 1 / 0.6 = 1.67 times. The present inventors have made various studies from such a viewpoint, and have completed a silicon nitride ceramics substrate that does not have a problem of durability such as strength in practical use.

In the present invention, the thermal resistance of the silicon nitride ceramic substrate can be measured, for example, as follows. First, using the apparatus shown in FIG. 1, the total thermal resistance (R _ALL ) is calculated from the temperature difference (° C.) between the transistor temperature T ₁ and the aluminum heat dissipation block temperature T ₂ with respect to the electric power (W) loaded on the transistor (R _ALL ). It is calculated using the formula 1). At that time, 16 × 20 mm is suitable as the size of the silicon nitride ceramics substrate.

Since the total thermal resistance (R _ALL ) can be decomposed as in the equation (2), the thermal resistance R _p of the silicon nitride ceramic substrate is given by the equation (3). _{R ALL = R Tr + R Gr1} + R P + R Gr2 ··· (2) R Tr: Thermal resistance R of the transistor (silicon) _Gr1: thermal resistance of the transistor side of the heat dissipation grease R _P: Thermal resistance R of the silicon nitride ceramic substrate _Gr2: aluminum radiating heat resistance of the block side of the radiating grease _{R P = R ALL - (R} Tr + R Gr1 + R Gr2) ··· (3)

Therefore, as shown in FIG. 2, the temperature difference between the transistor temperature T ₃ and the aluminum heat radiation block temperature T ₄ is measured by using a device in which the parts are assembled without sandwiching the silicon nitride ceramics substrate, and the silicon nitride nitride is measured. Calculate the thermal resistance of parts other than the base ceramic substrate. The difference between the two is the required thermal resistance of the silicon nitride ceramic substrate. In this case, the thermal resistance of the parts other than the silicon nitride ceramic substrate may be measured by using a material that can be sufficiently ignored, for example, a copper plate having a sufficiently thin thickness, and the value may be used as a substitute.

However, when forming a copper circuit by thinning the silicon nitride ceramics substrate in this way, the strength of the whole becomes small, so that the reaction force against the thermal stress due to the difference in thermal expansion between the copper plate and the silicon nitride ceramics substrate. Becomes weak and there is a problem with reliability. However, when such a problem becomes a reality, room temperature bending strength is 700MP.
The problem can be solved by using a silicon nitride ceramics substrate having a or more, particularly 900 MPa or more.

The silicon nitride ceramic substrate used in the present invention is a silicon nitride sintered body containing 90% by weight or more of silicon nitride and 10% by weight or less of auxiliary components such as alumina, magnesia and yttria. Although desirable, the present invention is not limited to this, and a sialon sintered body in which this composition contains an aluminum component in an amount of about 1 to 20% by weight in terms of aluminum nitride may be used. Such a silicon nitride ceramic substrate is prepared by mixing raw material powders in water or an organic medium such as methanol, ethanol, or 1,1,1-trichloroethane to prepare a slurry, forming a green sheet, and removing the binder. After that, it can be manufactured by sintering at a temperature of 1700 ° C. or higher in an inert atmosphere.

As the structure of the ceramic substrate having the copper circuit of the present invention, for example, the silicon nitride ceramic substrate has a thickness of 0.3 mm, and the copper plate on the circuit side has a thickness of 0.3 to
When the thickness is 0.5 mm, the thickness of the back copper plate is 0.1 to 0.2
It is preferably 5 mm. As the copper plate, an oxygen free copper plate, a tough pitch copper plate or the like is used. As a method of joining the silicon nitride ceramics substrate and the copper plate, there is no problem with either the active metal brazing method or the DBC method, but the active metal brazing method having a low joining temperature is more preferable.

As a method of forming a copper circuit, it is desirable to bond a copper plate to the entire surface of the silicon nitride ceramics substrate in advance and then perform etching with ferric chloride or cupric chloride. At this time, it is desirable to change the etching speed, processing temperature, chlorine ion concentration, etc. according to the thickness of the copper plate. As the shape of the pattern, it is desirable that the length of the continuous pattern is shorter than the length of the silicon nitride ceramics substrate so that the difference in thermal expansion from the silicon nitride ceramics substrate is made as small as possible.
Also, regarding the size of the silicon nitride ceramics substrate, the thermal stress applied to the end face of the substrate increases in proportion to the length of the substrate, and thus it is desirable that the size is as small as possible.

[0020]

EXAMPLES The present invention will be specifically described below with reference to Examples and Comparative Examples.

Examples 1 and 2 After preparing a slurry containing 91% by weight of silicon nitride powder, 5% by weight of yttria powder and 4% by weight of alumina powder, a green sheet was formed, and the binder was removed. Size 16 x 2 after pressureless sintering at temperature
A silicon nitride ceramic substrate having a thickness of 0 mm and a thickness of 0.2 mm or 0.3 mm was manufactured, and its thermal resistance and thermal conductivity were measured. The thermal resistance was measured by assembling the device of FIGS. 1 and 2,
The thermal conductivity was measured by the laser flash method.

75 parts by weight of silver powder, 25 parts by weight of copper powder, 20 parts by weight of zirconium powder, 15 parts by weight of terpineol and 1.5 parts by weight of a solid solution of a toluene solution of polyisobutyl methacrylate as an organic binder are mixed. A brazing material paste was prepared and applied on both surfaces of the silicon nitride ceramic substrate obtained above by screen printing. The coating amount (after drying) at that time was 6 to 8 mg / c.
It was set to m ² .

Then, a thickness of 0.5 m is formed on the surface of the silicon nitride ceramic substrate coated with the brazing paste.
m copper plate and the back surface of the copper plate having a thickness of 0.2 mm are placed in contact with each other, and then placed in a furnace and heated in a high vacuum of 1 × 10 ⁻⁵ Torr at a temperature of 900 ° C. for 30 minutes, and then at 2 ° C. / A joined body was manufactured by the active metal brazing method by cooling to room temperature at a cooling rate of about a minute.

Next, a UV curing type etching resist is applied to the circuit pattern by screen printing on the copper plate of the above-mentioned bonded body, and then an etching treatment is carried out using a cupric chloride solution to dissolve unnecessary parts of the copper plate. After removal, the etching resist was stripped with a 5% caustic soda solution. In the bonded body after the etching treatment, there is a residual unnecessary brazing material and an active metal component and a reaction product of the silicon nitride ceramics substrate between the copper circuit patterns.
It was immersed in a 10% ammonium fluoride solution at 0 ° C for 10 minutes.

Example 3 85% by weight of silicon nitride powder and 13 aluminum nitride powder
A ceramic substrate having a copper circuit was manufactured in the same manner as in Example 1 except that a sialon slurry having a composition of wt% and yttria powder of 2 wt% was prepared.

A heat cycle (thermal shock) test was conducted on the ceramic substrate having a copper circuit obtained through these series of treatments. The heat cycle test is in the air, −
After holding at 40 ° C. for 30 minutes, it was left at 25 ° C. for 10 minutes, further held at 125 ° C. for 30 minutes, and then left at 25 ° C. for 10 minutes as one cycle. For evaluation, dozens of samples were prepared for each example of each example, and a heat cycle test was immediately performed. Then, the state of each sample is observed every 3 cycles, and if even one sample has peeled copper plate, the number of cycles at that time is taken as the copper plate peeling start number, The heat cycle resistance was evaluated. The results are shown in Table 1.

[0027]

[Table 1]

[0028]

According to the present invention, a silicon nitride-based ceramic substrate suitable as a substrate for thyristors is obtained, and a ceramic substrate having a copper circuit manufactured using this silicon nitride-based ceramic substrate is The durability against history, that is, the heat shock resistance and the heat cycle resistance are significantly improved.

[Brief description of drawings]

FIG. 1 is an explanatory diagram for measuring the thermal resistance of a silicon nitride ceramics substrate.

FIG. 2 is an explanatory diagram for measuring the thermal resistance of a silicon nitride ceramics substrate.

[Explanation of symbols]

1 transistor (“2SC-3042” bottom area: 16
X 20 mm ² ) 2 Silicon nitride ceramic substrate (16 x 20 mm x
(Thickness) 3 Heat dissipation grease on transistor side 4 Heat dissipation grease on aluminum heat dissipation block 5 Aluminum heat dissipation block (water cooling type)

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI technical display location H05K 1/03 B 7011-4E // H02M 1/00 R 8325-5H C04B 35/58 302 C ( 72) Inventor Katsunori Terano 1 Shinkai-cho, Omuta City, Fukuoka Prefecture Inside the Omuta Factory of the Electrochemical Industry Co., Ltd.

Claims (2)

[Claims] 1. A silicon nitride ceramic substrate having a thermal resistance of 0.20 ° C./W or less. 2. The silicon nitride ceramics substrate according to claim 1, wherein a copper circuit is formed on one surface, and a copper plate having an area substantially equal to that of the silicon nitride ceramics substrate is bonded to the opposite surface. A ceramic substrate having a copper circuit.